1. Field of the Invention
The present invention relates to nonreciprocal circuit elements, such as isolators and circulators, used for communication apparatuses, such as mobile phones.
2. Description of the Related Art
Lumped-constant isolators are high-frequency components which allow signals to pass in the transmission direction without any loss and prevent signals from passing in the reverse direction, and are used for transmission circuits of mobile communication apparatuses, such as mobile phones. FIG. 10 is an assembly view showing the structure of such an isolator.
A conventional isolator 100 shown in FIG. 10 includes a magnetic assembly 110 including a magnetic body 104 composed of yittrium-iron-garnet (YIG) and a central conductor 105; a capacitor substrate 120 connected to the central conductor 105; and a permanent magnet 130, all of which are stored in an upper metal case 141 and a lower metal case 142 which also function as magnetic yokes. A DC bias magnetic field is applied to the magnetic assembly 110 by the permanent magnet 130. The conventional isolator 100 is approximately 5 by 5 mm square when used for a small mobile communication apparatus, such as a mobile phone.
The central conductor 105 includes a common electrode (not shown in the drawing) provided along the lower surface of the magnetic body 104, a first central conductor 106, a second central conductor 107, and a third central conductor 108, the central conductors 106, 107, and 108 extending radially in three directions from the common electrode and being folded along the upper surface of the magnetic body 104. The central conductors 106, 107, and 108 are joined together at the common electrode, which corresponds to a ground section. Although not shown in the drawing, the central conductors 106, 107, and 108 are insulated from each other by insulating sheets at the front surface of the magnetic body 104.
The capacitor substrate 120 is provided with matching capacitors corresponding to the individual ports, which will be described below.
The ends of the three central conductors 106, 107, and 108 protrude laterally from the magnetic body 104 to form the individual ports. Each port is connected to each matching capacitor, and one of the ports is connected to a terminator via the matching capacitor.
In the conventional isolator, the saturation magnetic flux density (4πMs) of YIG, which constitutes the magnetic body 104, changes by 24% to 34% in the operating temperature range (specifically, −35° C. to +85° C.), i.e., the saturation magnetic flux density at 85° C. is 24% to 34% lower than that at −35° C. In particular as the temperature increases, the decreasing rate of the saturation magnetic flux density increases. On the other hand, the residual magnetic flux density (Br) of the permanent magnet 130 changes only by 21% to 22%, i.e., the residual magnetic flux density at 85° C. is 21% to 22% lower than that at −35° C.
In the conventional isolator, there is a large difference between the temperature coefficient of the saturation magnetic flux density of the magnetic body 104 and the temperature coefficient of the residual magnetic flux density of the permanent magnet 130. Therefore, although a proper DC bias magnetic field is applied to the magnetic body 104 at low temperatures, as the temperature increases, the decreasing rate of the saturation magnetic flux density of the magnetic body 104 increases, and thereby the residual magnetic flux density of the permanent magnet 130 relatively increases. Consequently, a strong DC bias magnetic field tends to be applied to the magnetic body 104 from the magnetic circuit comprising the magnetic yokes (the upper case 141 and the lower case 142) and the permanent magnet 130. With such a tendency, the isolation peak deviates from the desired value, and the isolation frequency characteristics change, resulting in an increase in the insertion loss and a decrease in signal transmission efficiency.
Additionally, the upper case 141 and the lower case 142 are composed of a material that is substantially pure iron, such as SPCC. The SPCC has a Curie point (Tc) of approximately 727° C. (1,000 K). In a material with a Curie point (Tc) of approximately 1,000 K, the saturation magnetic flux density (4πMs) changes only by approximately 1.0%. Therefore, it is not possible to compensate for the difference between the temperature coefficient of the saturation magnetic flux density of the magnetic body 104 and the temperature coefficient of the residual magnetic flux density of the permanent magnet 130.